1. Field of the Invention
The invention relates to an integrated circuit process, and particularly relates to a box-in-box (BiB) overlay mark used for overlay measurement in IC processes.
2. Description of Related Art
There are many different designs for the overlay marks for overlay measurement. A representative design is the box-in-box design, which typically includes an inner box including x- and y-directional linear patterns of the photoresist layer for defining the current layer, and an outer box including x- and y-directional linear patterns of the previous layer. The previous layer may have been defined to form trenches/openings therein or defined into line/block patterns. The photoresist layer may have been defined to form trench/opening patterns therein or defined into line/block patterns.
After the linear patterns are transferred to the substrate, followed by a metal process, defectivity concern may be raised. Hence, a design rule has been established to limit the size of largest trench that can be allowed prior to some metal process.
However, for certain processes, the design rule may become an issue. One example is how to register a chop mask to a pitch reduction layer for a DRAM word-line process. The outer box is defined by oxide spacer, but not transferred to the substrate yet. The chop layer can have photoresist on the outer box, so the outer box will never be transferred into the substrate and there is no concern for the outer box. The problem comes from the current layer (chop layer, photoresist), in which trenches have to be formed as the inner box for the overlay measurement. Limited by the lithography tool and metrology tool resolution limit, the dimension of the trenches may have to exceed the limit defined by the design rule. A solution is to add dense parallel narrow trenches in the previous layer under the inner-box photoresist patterns.
An example of such a BiB overlay mark is illustrated in FIGS. 1A to 1C, wherein FIG. 1B and FIG. 1C illustrate cross-sectional views along the line B-B′ and along the line C-C′ in FIG. 1A, respectively.
The overlay mark 10 includes dense parallel narrow line/trench patterns 110 in the previous layer that include alternately arranged line patterns 110a and trench patterns 110b orientated in the x- or y-direction in the entire region (inner and outer box regions), x-directional and y-directional broad trenches 112 constituting the outer box in the previous layer, and x-directional and y-directional broad trenches 122 constituting the inner box that are defined in the photoresist layer 120 defining the current layer.
However, in such a design, the two sidewalls of each inner-box trench 122 parallel with the line/trench patterns 110a/110b will possibly see different optical impacts from the line patterns 110a due to imperfect alignment, and/or snap to the nearest line patterns 110a, so that the position determination accuracy thereof is adversely affected and the overlay measurement accuracy is lowered.